1. Field of the Invention
The present invention relates to the field of transconductors, and more particularly to linear transconductors.
2. Prior Art
One of the basic building blocks in analog electronics is the transconductor, permitting the designer to convert an input voltage signal into a current-mode signal. The differential pair is the most common, but suffers from an inherently small linear input range. In order to increase the linear input range, feedback in the form of emitter degeneration is typically used. This form of feedback improves linearity at the expense of gain and noise performance. To reduce power supply consumption, an alternative linearization technique was developed, known as the multitanh. The multitanh approach involves using a pair of parallel connected differential pairs with intentional symmetrical input offsets forced by emitter area ratios. Others have used more than two differential pairs with varying offsets to attempt to achieve a wider linear input range. However, this method suffers significant degradation in performance when process variation of the devices is considered.
The standard degenerated transconductor is shown in FIG. 1. It is the most common open loop form of linearization. As shown, resistors R1 and R2 (R1=R2) in the emitter circuits of transistors Q1 and Q2 (of equal size) broaden the transconductance versus differential input curve of the differential pair, at the expense of gain and noise, to provide a wider range of differential input for a given error in linearity. The collector circuits of transistors Q1 and Q2 of FIG. 1, as well as the collector circuits of transistors Q3, Q4, Q5 and Q6 of FIG. 2, are shown as being coupled by resistors R5 and R6, respectively, and resistors R7 and R8, respectively, to VCC. In a typical circuit, the collector circuits would likely differ however. By way of one example only, in a typical operational amplifier, the current in one collector circuit is mirrored to the other collector circuit, which thus becomes a summing point for the difference in currents in the input transistors, converting the differential voltage input to a single ended current output for the input stage of the operational amplifier.
The multitanh circuit shown in FIG. 2 has found increased use recently due to the continued push to lower supply voltages and lower power consumption. With transistor Q3 larger than transistor Q4, the peak transconductance of this differential pair is skewed toward a negative differential input voltage (Vinn greater than Vinp). With transistor Q5 larger than transistor Q6, the peak transconductance of the second differential pair is skewed toward a positive differential input voltage (Vinp greater than Vinn). The net effect is to flatten and broaden the transconductance of the parallel combination of the transistor pairs. The 4:1 area ratio flattens the transconductance characteristic as much as possible without introducing a minimum in transconductance (Gm) at zero differential input.
While the multitanh extends the linear range beyond that of an undegenerated differential pair, it still is only good for about 40 mvpp of input signal before the transconductance falls more than 1% from the peak. The use of more than two differential pairs with offsets of the additional pairs forced by higher emitter area ratios have also been proposed to further flatten the transconductance curve. However, the inventor has found these suffer marked performance degradations over processing conditions due to a sensitivity to physical emitter resistance in the bipolar junction transistors.